1. Field of the Invention
The present invention relates to a method and circuit for improving a quality of display on an LCD screen. More particularly, the present invention relates to a method of and circuit for synchronizing a vertical synchronization signal of the video signal supplied to an LCD system with an oscillation signal produced by a burst mode inverter in order to suppress interference noise appearing on an LCD screen.
2. Description of the Related Art
Producing an image using a liquid crystal display (LCD) is well known. LCD's are widely used for various applications including monitors, computers, televisions, and so on. The liquid crystal panel usually comprises a large number of individual liquid crystal pixel elements. Those pixel elements are beneficially organized in a matrix comprising pixel rows and pixel columns. Typically, the video signal is applied to the pixel elements by rows. A periodic vertical synchronization signal of the video signal is provided to scan all rows repeatedly. In one period of the vertical synchronization signal, all rows are successively scanned once. The number of times a pixel element of a column is scanned in a second is the frequency of the vertical synchronization signal.
An LCD system uses a backlight to illuminate the liquid crystal panel so as to produce an image. A backlight includes lamps, such as cold cathode fluorescent lamps (CCFL), for producing light. These lamps are typically powered by a DC-to-AC inverter. The inverter in turn is powered by another power source such as an LCD power supply. It converts a DC voltage to a high AC voltage required to drive the fluorescent lamp. One type of the DC-to-AC inverter is called burst mode inverter. The burst mode inverter generates oscillation waves to produce a square pulsating signal for dimming of the lamps. During the square pulses, the lamps are lighted through current. For example, FIG. 1 illustrates a voltage oscillation signal Vsawtooth, which is a sawtooth waveform, generated by a burst mode inverter, a burst pulsating signal resulting from the sawtooth wave, and the current signal Ilamp flowing through the lamp. The burst signal is produced according to the oscillation signal Vsawtooth compared with respect to a reference DC voltage Vr. When the voltage of the oscillation signal is above the reference voltage Vr, the burst signal is high. Otherwise the burst signal is low. Afterwards, the current lamp through the lamp is forced during the pulses of the burst signal. Besides, all lamps are driven in the same period and in synchronization with each other to reduce interference noise into the circuit of LCD from lamp or cables affecting display quality.
However, conventional LCD systems, especially multi-lamp LCD systems, still have interference noise problem. When the burst signal frequency of the burst mode inverter is equal or near the frequency of the vertical synchronization signal or its harmonics, a large interference noise will be generated periodically. This periodic noise will appear and disappear on the display screen and generates a so-called “ripple phenomenon”. For instance, if the frequency of the vertical synchronization signal is 60 Hz, when the burst signal frequency of Inverter is 120, 180, 240 Hz or higher (harmonics of 60 Hz), significant noise will result. The burst signal frequency is often preferably set to be about 150 Hz or higher to avoid being close to the harmonics or flicker perceived by human eyes. However, the tolerance of the burst mode frequency could be big due to tolerances of temperature-dependent components, including especially capacitors, the controller or operational amplifier of the inverter. Therefore, the burst mode frequency is not very stable and the noise problem “ripple phenomenon” is still a concern. For the forgoing reasons, there is a need for solving the ripple phenomenon problem due to interference noise.